Easterby, 36, quit professional rugby at the end of last season but he has been a regular this season for Otley in England’s National League Two.
Leinster coach Michael Cheika said: “Guy has huge experience in the game and was a popular member of the squad up until his retirement and I have no doubt he will give us the depth in the scrum-half position that we need at this important part of our year.
“He is a proven leader and it is also an added boost that he is familiar with the set-up.
“I would also like to express my thanks to Otley for their understanding and co-operation in facilitating the move.”
Easterby will be eligible to play in both the Heineken Cup and the Magners League.